ACTINOZOA. 



255 



cleavage of the parent polype, the process of division commencing with 

 a constriction of the oral disc, which gradually deepens till the original 

 animal becomes divided into two more or less completely separate 

 polypes, which, however, remain connected proximally. The process 

 is not sharply distinguishable from gemma- 

 tion, and the form of corallum produced there- 

 by varies greatly in different cases. In many 

 instances the corallites produced by fission 

 may be divided for a considerable distance, 

 remaining connected by the basal portion of 

 the original polype only, the resulting corallum 

 being of a " caespitose ;: or tufted form, and 

 consisting of short diverging pairs of branches, 

 of which each pair represents the division of a 

 single corallite. In other cases, the separation 

 of the secondary polypes is very imperfect, and 

 the corallum tends to assume a massive or 

 lamellar form. In such cases the calices form continuous rows, with 

 more or less clearly distinguishable centres, and the calicine furrows 

 are often winding and contorted, as seen in the familiar Brain -corals 

 {Mceandrina). 



In such corals as Chcetetes (fig. 137), the fissiparous development of 

 the new corallites can be generally recognised in sections without 



36. — Aulopora serpens. 

 Devonian. 



Fig. 137. — Sections of Chcetetes septosus, from the Carboniferous rocks, enlarged to show fission 

 of the corallites. A, Cross-section of a few corallites, some of which show the commencing fission 

 of a tube by the development of an internal longitudinal partition (/); b, Vertical section, show- 

 ing a single corallite (c) splitting into two. (Original.) 



difficulty. The commencing fission of a corallite is marked by the de- 

 velopment of an internal vertical partition, which is at first incomplete, 

 and which therefore appears in transverse sections as an inwardly directed 

 tooth-like process (fig. 137, a, p\ looking like a septum. A correspond- 

 ing partition is then developed opposite to the first one, and by the 

 inward growth and final junction of these, the original tube is ulti- 

 mately divided into two. In longitudinal sections (fig. 137, b), the fis- 

 sion of a corallite is marked by the appearance of a longitudinal parti- 

 tion in the interior of a tube, which thus becomes divided into two — the 

 two new corallites being at first of small size, but gradually assuming 

 their full dimensions. 



